This invention relates to a method for navigating magnetic devices in the body, and in particular to a method for safely and efficiently navigating magnetic devices in the body using a moveable source magnet outside the body.
The navigation of magnetic medical devices, such as magnet-tipped guide wires, catheters, endoscopes, or other instruments, with a movable source magnet presents several difficulties in ensuring that the movement of the medical device is as the physician expects and intends. The difficulties arise for several reasons, including the lag between the direction of the magnetic field applied by the magnet and the actual direction of the tip of the medical device, and xe2x80x9cconingxe2x80x9d of the tip of the medical device as it deviates from the intended plane of the turn as it turns.
According to one aspect of the invention, navigation of a magnet-tipped medical device takes into account the lag between the direction of the magnetic field applied by the source magnet and actual direction of the magnet tip. It is known that the magnet tip will lag the exact direction of the magnetic field at its location by some finite amount. This lag is the result of a restoring torque due to the stiffness of the attached device (e.g., the guidewire, catheter, endoscope, or other device to which the magnetic element is associated).
This creates an ambiguity between the applied magnetic field and the actual direction of the magnet tipped device that can interfere with safe and efficient navigation. The way this turn angle ambiguity is removed is to provide a lead angle for the magnetic field which accounts for the restoring, or turn-resisting, torque of the attached medical device. According to one embodiment of this invention, information about the restoring stiffness of the medical device to which the magnet is attached (e.g., a guidewire, catheter, endoscope or other device) is included in a computer program controlling the navigation. Information of about the desired angle of turn and the desired radius (sharpness) of the desired turn can reside either in a lookup table or equation programmed in the computer. This information depends upon the properties of the device with which the magnet tip is associated, and thus will be different for each different medical device. Given the magnitude of the moment of the tip magnet and this restoring torque, which is set equal to xcex93, the value of B needed to achieve the required angle xcex8 will follow.
According to a second aspect of this invention, it is desirable to make turns in such a way as to maintain the magnet tip of the medical device in the same plane as the initial direction and the desired final direction, avoiding the problem of xe2x80x9cconingxe2x80x9d in which the magnet tip swings out of the plane of the turn. This is particularly important when the navigation is through the parenchyma, although even when navigating through body lumens, such as blood vessels, maintaining planarity during the turn can be important. While the movement of the source magnet usually accurately aligns the tip of the medical device in the desired final direction, the movement of the magnet does not necessarily move the tip in the desired plane.